type texenv_pname =
  | GL_TEXTURE_ENV_MODE
  | GL_TEXTURE_LOD_BIAS
  | GL_COMBINE_RGB
  | GL_COMBINE_ALPHA
  | GL_SRC0_RGB
  | GL_SRC1_RGB
  | GL_SRC2_RGB
  | GL_SRC0_ALPHA
  | GL_SRC1_ALPHA
  | GL_SRC2_ALPHA
  | GL_OPERAND0_RGB
  | GL_OPERAND1_RGB
  | GL_OPERAND2_RGB
  | GL_OPERAND0_ALPHA
  | GL_OPERAND1_ALPHA
  | GL_OPERAND2_ALPHA
  | GL_RGB_SCALE
  | GL_ALPHA_SCALE
  | GL_COORD_REPLACE
